The transfer function of any two-port device or four-terminal network, no matter whether active or passive, is known to depend more or less on the temperature of the room wherein it operates. Variations in the characteristic parameters of the single individual components result in an amplitude and phase variation in the transfer function of the two-port device. This phenomenon can be of great importance in a number of cases and particularly at high frequency.
The phase variation of the transfer function often gives rise to problems more serious than the variation of the modulus as its compensation is more difficult.
In case of distributed-parameter networks phase variations are chiefly determined by:
(1) variation of the circuit geometry due to thermal expansion of the material;
(2) variation of the dielectric constant of the medium through wich a high-frequency signal propagates, with consequent variation in the propagation velocity.
Know methods able to effect an accurate compensation, designed to keep phase variation within a few electrical degrees with temperature variation within several degrees Celsius, are basically two.
A first method is that of using special materials with near-zero coefficients of both thermal expansion and dielectric-constant variation.
The second method is that of introducing into the network structure such mechanical variations with the temperature that they may compensate the overall phase variation.
In the first case the materials needed must meet a number of different requirements. More particularly, besides having extremely low thermal coefficients both of expansion and of dielectric-constant variation, they must present good high-frequency electrical characteristics chiefly in the field of microwaves, and suitable mechanical characteristics depending on their use. In addition, the production of these materials is very expensive as it requires sophisticated technologies in order to minimize, disparities in the product properties.
In the second case, phase variations in the transfer function of the two-port device are compensated by a mechanical variation in the stucture wherein propagation takes place. In this manner a variation of distributed network parameters takes place so as to cause a phase variation in a direction opposite that due to temperature influence on the circuit. However, this technique can prove rather critical in the initial adjustment, the degree of reliability is lower owing to the presence of mechanical parts in motion, and direct integration of the compensated device may be difficult in more complex systems.